As the proliferation of wireless communication continues, the demand for speed and reliability over a wireless connection is constantly rising. In recent years, technologies such as multiple-input-multiple-output (MIMO) and carrier aggregation have been used to increase both speed and reliability of a wireless connection. At a high level, MIMO and carrier aggregation allow multiple data streams to be simultaneously transmitted and/or received by a device. These data streams are generally transmitted and/or received at different frequencies and then separated by the device to obtain the data therein. While this process is generally straightforward when the frequencies used to transmit the data streams are separated by a large frequency delta, it becomes significantly more complex when the frequency delta between two data streams is below a certain amount. This is due to the filtering circuitry that is used to separate the data streams. While it is relatively easy to design a filter that separates signals with frequencies that are far away from one another, it is generally very hard to do so for signals with frequencies that are close together, especially while maintaining desirable performance parameters of the filtering circuitry.
FIG. 1 shows conventional radio frequency (RF) front end circuitry 10 that may be used for MIMO and/or carrier aggregation. The conventional RF front end circuitry 10 includes a first antenna 12A, a second antenna 12B, a first diplexer 14A coupled to the first antenna 12A, a second diplexer 14B coupled to the second antenna 12B, front end switching circuitry 16 coupled to the first diplexer 14A and the second diplexer 14B, filtering circuitry 18 coupled to the front end switching circuitry 16, and transceiver circuitry 20 coupled to the filtering circuitry 18. When receiving multiple RF receive signals via MIMO and/or carrier aggregation, the first antenna 12A is generally used to receive primary RF receive signals, while the second antenna 12B is generally used to receive secondary RF receive signals. However, this configuration may be swapped by the front end switching circuitry 16, which may connect the first antenna 12A and the second antenna 12B to the filtering circuitry 18 and the transceiver circuitry 20 such that primary RF receive signals are received via the second antenna 12B and secondary RF receive signals are received via the first antenna 12A. The first diplexer 14A and the second diplexer 14B generally separate RF receive signals from the first antenna 12A and the second antenna 12B, respectively, based on the frequency thereof. For example, the first diplexer 14A and the second diplexer 14B may separate RF receive signals into low-band RF receive signals and high-band RF receive signals, separately delivering these signals to the front end switching circuitry 16. In some cases, the first diplexer 14A and the second diplexer 14B may be triplexers, quadplexers, or any order n-plexers in order to increase the granularity of separation between the RF receive signals.
The front end switching circuitry 16 connects the first diplexer 14A and the second diplexer 14B to one or more additional filters in the filtering circuitry 18 and an appropriate receiver in the transceiver circuitry 20. The filtering circuitry 18 further filters the RF receive signals that were separated by the first diplexer 14A or the second diplexer 14B, and may perform additional separation of multiple RF receive signals that were not separated by the diplexers 14. The transceiver circuitry 20 generally amplifies the separated RF receive signals and performs any necessary decoding to obtain the data therefrom.
As discussed above, the process for effectuating MIMO and/or carrier aggregation is generally relatively straightforward when the RF receive signals are separated by a large frequency delta. For example, if a first RF receive signal is a high-band Long Term Evolution (LTE) signal and a second RF receive signal is a low-band LTE signal, these signals will be easily separated by the first diplexer 14A and the second diplexer 14B using conventional filter designs and then separately routed to the filtering circuitry 18 for additional cleanup and the transceiver circuitry 20 for amplification and decoding. While early MIMO and/or carrier aggregation configurations focused on pairing multiple RF receive signals separated by relatively large frequency deltas, it may also be desirable to perform MIMO and/or carrier aggregation for RF receive signals with frequencies that are close to one another.
Conventionally, the first diplexer 14A and the second diplexer 14B have been designed as either lumped element filters or acoustic filters. While lumped element filters are generally able to achieve a high bandwidth, the selectivity of such filters is quite poor due to the slow roll-off thereof. To illustrate this, FIG. 2A shows a filter response of a conventional lumped element diplexer. A first signal path in the conventional diplexer provides a first bandpass filter response in order to pass signals within a first frequency band FB1 while a second signal path in the conventional diplexer provides a second bandpass filter response in order to pass signals within a second frequency band FB2. The first bandpass filter response is illustrated by a first line 22, while the second bandpass filter response is illustrated by a second line 24. While the conventional lumped element diplexer will be suitable for isolating a large portion of signals within the first frequency band FB1 and the second frequency band FB2, as the signals approach the lower end of the first frequency band FB1 or the upper end of the second frequency band FB2, there is significant cross-contamination of the signals. In other words, a large portion of a signal within an upper portion of the second frequency band FB2 will be passed along with signals in the lower portion of the first frequency band FB1, and vice versa. Put simply, the conventional lumped element diplexer cannot properly separate signals that are within a predetermined frequency delta of one another due to the poor selectivity thereof.
FIG. 2B shows a filter response of a conventional acoustic diplexer. A first signal path in the conventional acoustic diplexer provides a first bandpass filter response in order to pass signals within a portion of the first frequency band FB1 while a second signal path in the conventional acoustic diplexer provides a second bandpass filter response to pass signals within a portion of the second frequency band FB2. The first bandpass filter response is illustrated by a first line 26, while the second bandpass filter response is illustrated by a second line 28. As shown in FIG. 2B, the isolation of the conventional acoustic diplexer is significantly improved over the conventional lumped element diplexer such that cross-contamination of signals within the separate frequency bands is reduced or eliminated altogether. However, the bandwidth of the conventional acoustic diplexer is significantly decreased such that the conventional acoustic diplexer is incapable of passing signals within the entirety of the first frequency band FB1 and the second frequency band FB2. While additional acoustic filtering elements may be used to achieve a desired pass-band, these additional acoustic filtering elements may be quite large and thus consume a large amount of space. In mobile communication devices where space is generally a primary design concern, this is not a practical approach. Accordingly, there is a need for improved RF filtering circuitry capable of separating signals for MIMO and/or carrier aggregation in a wireless communication device.